Fix modification of members of object with const handle (#7433)

Signed-off-by: Kamil Danecki <kdanecki@internships.antmicro.com>

src/V3Width.cpp

@@ -218,6 +218,7 @@ class WidthVisitor final : public VNVisitor {
     bool m_underFork = false;  // Visiting under a fork
     bool m_underSExpr = false;  // Visiting under a sequence expression
     bool m_underPackedArray = false;  // Visiting under a AstPackArrayDType
+    bool m_underMemberSel = false;  // Viting under a MemberSel
     bool m_hasNamedType = false;  // Packed array is defined using named type
     AstNode* m_seqUnsupp = nullptr;  // Property has unsupported node
     bool m_hasSExpr = false;  // Property has a sequence expression
@@ -2924,8 +2925,8 @@ class WidthVisitor final : public VNVisitor {
             }
         } else if (nodep->access().isWriteOrRW() && nodep->varp()->isConst() && !m_paramsOnly
                    && (!m_ftaskp || !m_ftaskp->isConstructor())
-                   && !VN_IS(m_procedurep, InitialAutomatic)
-                   && !VN_IS(m_procedurep, InitialStatic)) {
+                   && !VN_IS(m_procedurep, InitialAutomatic) && !VN_IS(m_procedurep, InitialStatic)
+                   && !m_underMemberSel) {
             // Too loose, but need to allow our generated first assignment
             // Move this to a property of the AstInitial block
             nodep->v3warn(E_CONSTWRITTEN, "Writing to 'const' data-typed variable "
@@ -3561,6 +3562,8 @@ class WidthVisitor final : public VNVisitor {
     void visit(AstMemberSel* nodep) override {
         UINFO(5, "   MEMBERSEL " << nodep);
         if (nodep->didWidth()) return;
+        VL_RESTORER(m_underMemberSel);
+        m_underMemberSel = true;
         UINFOTREE(9, nodep, "", "mbs-in");
         userIterateChildren(nodep, WidthVP{SELF, BOTH}.p());
         UINFOTREE(9, nodep, "", "mbs-ic");
@@ -3709,6 +3712,11 @@ class WidthVisitor final : public VNVisitor {
                         VL_DO_DANGLING(pushDeletep(nodep), nodep);
                         return true;
                     }
+                    if (nodep->access().isWriteOrRW() && varp->isConst()) {
+                        nodep->v3warn(E_CONSTWRITTEN, "Writing to 'const' data-typed variable "
+                                                          << nodep->prettyNameQ()
+                                                          << " (IEEE 1800-2023 6.20.6)");
+                    }
                     nodep->dtypep(foundp->dtypep());
                     nodep->varp(varp);
                     nodep->didWidth(true);

test_regress/t/t_const_handle.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of either the GNU Lesser General Public License Version 3
+# or the Perl Artistic License Version 2.0.
+# SPDX-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_const_handle.v

@@ -0,0 +1,25 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Antmicro
+// SPDX-License-Identifier: CC0-1.0
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
+// verilog_format: on
+
+class Cls;
+  int x;
+endclass
+
+module t;
+  initial begin
+    const automatic Cls cls = new;
+    cls.x = 1;
+    `checkd(cls.x, 1);
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule

test_regress/t/t_const_handle_bad.out

@@ -0,0 +1,14 @@
+%Error-CONSTWRITTEN: t/t_const_handle_bad.v:15:9: Writing to 'const' data-typed variable 'x' (IEEE 1800-2023 6.20.6)
+                                                : ... note: In instance 't'
+   15 |     cls.x = 1;
+      |         ^
+                     ... For error description see https://verilator.org/warn/CONSTWRITTEN?v=latest
+%Error-CONSTWRITTEN: t/t_const_handle_bad.v:16:10: Writing to 'const' data-typed variable 'x' (IEEE 1800-2023 6.20.6)
+                                                 : ... note: In instance 't'
+   16 |     cls2.x = 1;
+      |          ^
+%Error-CONSTWRITTEN: t/t_const_handle_bad.v:18:5: Writing to 'const' data-typed variable 'cls' (IEEE 1800-2023 6.20.6)
+                                                : ... note: In instance 't'
+   18 |     cls = cls2;
+      |     ^~~
+%Error: Exiting due to

test_regress/t/t_const_handle_bad.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of either the GNU Lesser General Public License Version 3
+# or the Perl Artistic License Version 2.0.
+# SPDX-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('vlt')
+
+test.lint(fails=True, expect_filename=test.golden_filename)
+
+test.passes()

test_regress/t/t_const_handle_bad.v

@@ -0,0 +1,21 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Antmicro
+// SPDX-License-Identifier: CC0-1.0
+
+class Cls;
+  const int x;
+endclass
+
+module t;
+  initial begin
+    const automatic Cls cls = new;
+    automatic Cls cls2 = new;
+    cls.x = 1;
+    cls2.x = 1;
+
+    cls = cls2;
+    cls2 = cls;
+  end
+endmodule